1. Field of the Invention
The present invention relates to a process for enhancing the recovery of sodium carbonate monohydrate from soda ash process liquors containing both sodium carbonate and sodium bicarbonate. More specifically, the invention relates to the conversion of sodium bicarbonate to sodium carbonate in a monohydrate production process.
2. State of the Art
Sodium carbonate, also known as soda ash, is one of the highest volume chemicals produced in the United States. The majority of soda ash produced in this country comes from trona or nahcolite ore deposits located primarily in Wyoming, California, and Colorado. The ore is dry mined or solution mined and processed to produce sodium carbonate. Both dry mining and solution mining techniques are well known to those skilled in the art.
A number of production processes have been used to extract sodium carbonate from ores such as trona and nahcolite and, more specifically, from solutions made from these ores. Most of the soda ash production processes in the United States today use a sodium carbonate monohydrate crystallizer as the last crystallization step in the production process. To avoid the unwanted precipitation of sodium sesquicarbonate in a monohydrate crystallizer, commercial processes in use today have preliminary steps designed to reduce the bicarbonate concentration in the feed stream. For example, dry mined ores must be essentially completely calcined to convert sodium bicarbonate to sodium carbonate before dissolving the calcinate and feeding the clarified liquor to a monohydrate crystallization process. In some solution mining processes, even after initially concentrating and CO2 stripping the solution mined brine, intermediate crystallization steps are required to produce a monohydrate crystallizer feed with sufficiently reduced bicarbonate concentration. For example, sodium sesquicarbonate can be crystallized from the CO2-stripped brine to further reduce the sodium bicarbonate concentration. Prior art processes also crystallize sodium carbonate decahydrate crystals from the partially CO2-stripped brine as an intermediate which must be melted prior to the monohydrate crystallization step.
Known sodium carbonate monohydrate crystallization processes used to produce soda ash feed a sodium carbonate and sodium bicarbonate containing solution to a monohydrate crystallizer to crystallize sodium carbonate monohydrate. A slurry of wet crystals and mother liquor forms within the crystallizer. A portion of the slurry is discharged from the monohydrate crystallizer and the wet crystals are separated from the mother liquor. The wet crystals are dried to yield a soda ash product. A portion of the mother liquor separated from the wet crystals is purged from the process and the remainder is recycled back to the monohydrate crystallizer. As known in the art, however, if the sodium bicarbonate concentration of the crystallizer feed solution is not sufficiently reduced, both sodium carbonate monohydrate and undesired sodium sesquicarbonate crystals can form in the monohydrate crystallizer. One such process is described in U.S. Pat. No. 6,228,335. Therefore, in order to guarantee formation of only sodium carbonate monohydrate crystals, other steps, such as intermediate crystallization steps or increasing the purge rate of the crystallizer purge stream are used to control the bicarbonate concentration in the mother liquor.
Unlike sodium carbonate monohydrate crystals, sodium sesquicarbonate crystals are long, thin, needle-like crystals, which are difficult to dewater. In addition, the sodium sesquicarbonate crystals are prone to breakage resulting in an undesirable, dusty soda ash product. The soda ash produced by calcination of sesquicarbonate also has an undesirably low bulk density compared to that made by drying sodium carbonate monohydrate. It is, therefore, preferable to produce only sodium carbonate monohydrate crystals in a sodium carbonate monohydrate crystallization process. An alkali-efficient process of producing sodium carbonate monohydrate crystals in a crystallizer by feeding solutions containing substantial sodium bicarbonate concentrations to a sodium carbonate monohydrate crystallization circuit without the need for intermediate crystallization steps is, therefore, desirable.
The present invention relates to a process for enhancing the recovery of sodium carbonate monohydrate crystals from solutions containing sodium carbonate and sodium bicarbonate. More specifically, the invention relates to the decomposition of sodium bicarbonate to sodium carbonate in a sodium carbonate monohydrate production process.
In one embodiment of the present invention, a process feed solution containing both sodium carbonate and sodium bicarbonate is subjected to CO2 stripping prior to feeding the stripped solution directly to the sodium carbonate monohydrate crystallizer. The process feed solution of this invention is typically brine recovered from solution mining ores such as nahcolite or trona, sometimes containing minor bicarbonate constituents such as wegscheiderite, and which may have already been concentrated and partially CO2 stripped prior to feeding this process. The process feed solution can also be formed by dissolving dry mined ore in a solution or by blending the dissolved dry mined ore with solution mined brine. Steam being used to strip CO2 from the process feed is desirably the vapors of triple effect crystallizers or those from a crystallizer using mechanical vapor recompression. By CO2 stripping the monohydrate crystallizer circuit feed solution, the yield of sodium carbonate monohydrate is improved and the amount of purge liquor sent to waste is reduced. More importantly, the yield of recovered sodium carbonate monohydrate is increased without cocrystallizing sesquicarbonate by maintaining the mother liquor composition slightly below the carbonate monohydrate/sesquicarbonate invariant point, and without the use of intermediate crystallization steps.
In a preferred embodiment of the present invention, a process feed solution containing both sodium carbonate and sodium bicarbonate is fed directly to a sodium carbonate monohydrate crystallizer to crystallize sodium carbonate monohydrate crystals. The process feed solution is typically brine recovered from a solution mining process, solution formed by dissolving dry mined ore, or a combination thereof. At least a portion of the mother liquor from the crystallizer is recycled through a CO2 stripping column to decompose some of the sodium bicarbonate in the mother liquor to sodium carbonate. A portion of the stripped mother liquor is returned to the crystallizer. Crystallizer mother liquor feeding the CO2 stripper is typically sourced either directly from an essentially crystal-free zone within the crystallizer body or from any of several solid/liquid separation devices (i.e., centrifuges, filters, cyclones) applied to a carbonate monohydrate slurry withdrawn from the crystallizer and commonly known to those skilled in the art. In order to prevent precipitation of salts from the CO2 stripping operation, it may be necessary to dilute the mother liquor feeding the CO2 stripper.
Stripping column conditionsxe2x80x94such as temperature, steam rate, liquor feed rate, and the likexe2x80x94are adjusted to control the amount of decomposition of sodium bicarbonate in the stripped mother liquor. By adjusting the amount of mother liquor sent to the CO2 stripper, a steady state composition within the crystallizer body can be maintained at just below the invariant point corresponding to the cocrystallization of sodium sesquicarbonate and sodium carbonate monohydrate. This guarantees the production of only sodium carbonate monohydrate crystals within the sodium carbonate monohydrate crystallizer.